The boy was grabbed by a Crocodile while swimming in a group. Photograph: Getty Images

Police are searching for a nine-year-old boy who was taken by a crocodile while swimming in a river in northern Australia.

The boy was grabbed by the animal as he swam with a group of people at Port Bradshaw, Northern Territory, on Saturday.

Although a group of adults tried to kill the 13ft-long crocodile with spears, it dragged the boy into deeper water and he has not been seen since.

The animal was spotted later on Saturday, but armed search parties were unable to shoot it dead.

"The crocodile was sighted a number of other times and two shots were fired in an attempt to kill it," Sergeant Alex Brennan of Nhulunbuy police told the Northern Territory News. "The crocodile's gone underwater and hasn't been seen since."

Police, parks and wildlife officers and rangers are continuing the search but are growing increasingly pessimistic about finding the boy alive. "It's close to 24 hours since the child went missing," Brennan said.

"At this stage there's been no sighting of the child, and we obviously hold grave concerns for his welfare."

Two weeks ago, a crocodile snatched a seven-year-old girl who was swimming at a remote waterhole in the Northern Territory. The crocodile was shot dead the next day and the girl's remains found inside.

Both saltwater and freshwater crocodiles were hunted to near extinction in Australia, but have become plentiful in the tropical north since they became protected by federal law in 1971.

The Northern Territory is estimated to have more than 80,000 saltwater crocodiles, which can grow up to 7 metres (23ft) long and are the world's largest reptile. They are far more likely to attack humans than are the smaller freshwater crocodiles that also live in the region.